In the related art, a battery pack in which a block is constructed by connecting one or a plurality of batteries in series and a plurality of the blocks are connected in series is equipped in a hybrid automobile and an electric automobile, and various methods are proposed which detect or estimate the state of charge (SOC) of the battery.
For example, WO99/61929 discloses a technique having a battery model in which a simulated SOC is determined as a tentative value of the SOC of the battery and a battery voltage is estimated in consideration of a change of the state of the battery along with the simulated SOC, wherein the simulated SOC is corrected so that the estimated battery voltage and the battery voltage which is actually measured are equal to each other, to estimate the actual SOC.
FIG. 13 shows a structure of a device disclosed in WO99/61929. A charge/discharge current value detected by a current detecting unit 10 is integrated by a simulated SOC estimating unit 14 and is added to an initial value of the SOC of the battery which is determined in advance, so that the simulated SOC which is a tentative value of the SOC is estimated. Based on the estimated simulated SOC, an electromotive force estimating unit 16 estimates a battery voltage corresponding to the simulated SOC. The battery voltage is an open end voltage Vocv of the battery. The open end voltage Vocv is estimated by determining in advance, for each battery, a map between the SOC and the open end voltage. Based on the charge/discharge current, a voltage change estimating unit 18 estimates a voltage change due to an internal resistance of the battery. In addition, a dynamic voltage change estimating unit 20 estimates a change of the battery voltage based on a change of the charge/discharge current of the battery. The output values of the electromotive force estimating unit 16, the voltage change estimating unit 18, and the dynamic voltage change estimating unit 20 are added by an adder 22, and an estimated value Vest of the battery voltage is calculated. The estimated battery voltage Vest is compared by a comparator 24 with the actual measured voltage Vmes of a battery detected by the voltage detecting unit 12, and the difference is supplied to an SOC correction amount calculating unit 26. The SOC correction amount calculating unit calculates an amount of correction which makes the estimated voltage Vest equal to the measured voltage Vmes, and outputs the amount of correction to an adder 28, to correct the simulated SOC.
The open end voltage Vocv is calculated by determining, for each battery, the map between the SOC and the open end voltage in advance. If the open end voltage can be detected with a high precision, the SOC can be estimated using the map. In the related art, a method is known in which the open end voltage Vocv is calculated by measuring the voltage and current of the battery and using the method of least squares. However, such a method causes an increase in amount of calculation, and consequently, increases the processing time and load of the process program. In addition, if the operation speed is increased in this state, heat generation occurs, which is disadvantageous in terms of reducing the size of the detecting device.